1. Field of the Invention
The present invention relates generally to direct current generators employing a thermionic emitter within an evacuated chamber, the chamber containing an alkali metal vapor at low vapor pressures. The present invention particularly relates to employing such an apparatus in combination with a non-electrical source of heat to generate a desired direct current.
2. Description of the Prior Art
From the early experiment of Becqueral, Guthrie, Edison and others, it was known that by heating a metal, it is possible to "evaporate" ions or electrons from the metal. This electron or ion emission is related to the temperature of the emitting metal. It has been found that at lower temperatures the thermionic emission from metals is predominantly positive but that at much higher temperatures the negative or electron emission rapidly surpasses the positive and becomes all-important. It is thought that the initial positive emission from metals is largely due to impurities and the positive emission is seen to decrease as a function of time even where the temperature of the metal is held such that predominantly positive emission would be expected.
It has been observed that some metals emit electrons much more copiously than others. A notable example is thorium, an observed film of which on tungsten gives very copious electron emissions at high temperatures. This high temperature electron emission generally follows the Richardson-Dushman equation: EQU I.sub.s =AT.sup.2 e.sup.-B/T
I.sub.s is the thermionic current density. A is a proportionality constant which for thoriated tungsten is about 3. T is the absolute temperature while e is the base of natural logarithms. B is a constant equal to the work function of the metal divided by the Boltzmann constant. For thoriated tungsten the value of B is approximately 30500. Thus, for all practical purposes, the generation of usable levels of current occurs only when the emitting metal has a temperature above 1500.degree. C.
It is a known practice in the prior art to dispel the electron cloud which tends to form immediately above the surface of the metal during this high temperature electron emission. Thermionic devices containing a low pressure gas are known in the prior art to reduce the space charge since the passage of electrons emitted by the thermionic emitter through the gas will produce ionization by colliding with the gas molecules. The ionization gives rise to more electrons and positive ions, the latter being attracted to the space charge where they neutralize the charge.
In the invention the cesium vapor acts to lower the work function of the nickel of the collector and to eliminate the space charge by ionizing when it comes into contact with the hot tungsten emitter. The ionization of the vapor by electron flow in the low pressure chamber is minor.
Prior art thermionic devices operate with a small spacing, typically 0.005 inch, between the emitter and the collector. However, with such close spacing, a virtual "short" is formed under large current flows, eliminating the potential difference between the emitter and the collector.
The problem, then, is to maintain closeness without suffering the elimination of potential between the emitter and the collector. The closer the spacing between emitter and collector, the more efficient the operation of the thermionic device. The present invention utilizes a conductor and diode network in direct connection between emitter and collector. In order to maintain a potential difference the diode network is inserted in the center of the connector and outside network is inserted in the center of the connector and outside of the vacuum area of the device away from the heat generated by the tungsten. In this operation the distance between the emitter and collector is great enough to create a high resistance so that current flow within the vacuum chamber is small. The current flow is from the emitter through the diode to the collector. By the use of the solid state diode network the potential difference between the emitter and collector is maintained at a constant voltage thereby enabling the power output of the thermionic device to increase.
Quite remarkably, the prior art has failed to appreciate that a large space charge within an area of high resistance can be used to remarkable advantage by leaving the tungsten with a deficiency of electrons resulting in a more positive charge on the tungsten. The large area separating the collector and emitter in the invention does not permit current flow but enhances the flow through the conductor and diode network connection of the emitter and collector.
Therefore, it is the object of the present invention to present a low voltage high current generator which creates dc power directly from a non-electrical source of heat. It is a further object of the present invention to enhance the current generation by providing a means for cooling the collector or anode of the thermionic current generator.
It is a further object of the invention to use at least one diode in the forward bias between the emitter and the collector.